As is known, a tire essentially comprises a carcass including at least one reinforcing ply, a couple of bead wires around which the ply edges are folded back in loop form after interposition of an elastomeric filling fixed to the radially external surface of each bead wire, a belting structure arranged as a crown around the torically formed carcass, a tread band wrapped around the belt, and a couple of axially opposed sidewalls, radially extending between the bead wires and the tread band.
Some kind of tires have the side portions of the tread band arranged over the radially external end of the sidewalls, while in other kinds the upper ends of the sidewalls are overlapped on the ends of the tread band.
A conventional tire manufacturing method substantially comprises the steps of wrapping at least one carcass ply around a cylindrical assembly drum having a radially external surface connected to corresponding axially external side surfaces, perpendicular to the drum rotation axis, through two curvilinear connecting surfaces, preferably arc-shaped in transversal cross-section, said ply having a width greater than that of the drum; turning down the edges of the ply radially inwardly onto the drum side surfaces, fixing the bead wires, optionally already assembled with the corresponding filling, against the carcass ply edges at said side surfaces of the drum, folding back the carcass ply edges in loop form radially outwardly around the bead wires and against the carcass ply, preferably until they reach and cover a portion of the ply lying on the cylindrical surface of the drum, wrapping around the side portions of the carcass ply, on the cylindrical portion, the corresponding sidewall of the tire with the axially external edge laterally protruding from the drum for a part sufficient to reach the bead wire zone, pressing the sidewall against the carcass ply to produce in this manner a semi-finished product commonly known as a cylindrical carcass sleeve.
Subsequently, the above mentioned sleeve is withdrawn from the assembly drum (or first-step drum) after contraction of the drum and fitted onto a toric forming drum of the expanding type (or second-step drum) to associate the same with the belt pack and the tread band which were separately preassembled on an auxiliary assembly drum.
To implement this second operating step, after fitting the sleeve onto the forming drum the belt/tread band assembly is placed around the sleeve in coaxial alignment with its center-line plane (or equatorial plane of the tire) and air is fed under pressure into the sleeve to torically form the same until the crown zone of the sleeve is brought against the belt pack.
Subsequently, the coupling between the sleeve and the belt/tread assembly is improved with appropriate pressing means to provide a green tire which will then be inserted in a mold to form the tread pattern and vulcanize the tire.
It is commonly known and in any case clear from the above that in the case of tires provided with sidewalls closed on the ends of the tread band the method described above requires modification to allow pressing of the tread band against the carcass ply before the sidewalls are overlapped on the side portions of the tread band.
In practice, the following procedure is followed. In the step of connecting the sidewalls to the cylindrical carcass sleeve an anti-adhesive element, usually a strip of sheet material (e.g. polyethylene), is preliminarily wound around the carcass ply towards each end of the drum for a portion of predetermined width so as to prevent adhesion of the ply to the entire surface of the sidewalls which are subsequently overlapped around each anti-adhesive element with their axially internal edge on said anti-adhesive element, while taking care that the sidewall portion between their axially external edge and the axially external edge of the anti-adhesive element be wide enough to ensure adhesion of the sidewall to the carcass in order to prevent relative sliding between the sidewall strip and the carcass ply during rotation of the assembly drum.
Next, after jointing together the ends of the sidewall portion deposited on the carcass and pressing the zone of the axially external edge of the sidewall against the carcass ply, said sleeve is removed from the assembly drum and fitted on the forming drum (i.e. the second-step drum), wherein the anti-adhesive element interposed between the sidewall and the carcass is withdrawn. This withdrawal is necessary to be able to associate the corresponding axially internal surface of the sidewalls with the carcass ply and over the side surfaces of the tread band.
The operation is manually or mechanically carried out primarily by gradually spreading apart the sidewalls from the carcass sleeve and then by removing the anti-adhesive element: preparation and subsequent rolling of the belt/tread assembly against the carcass ply are then performed and, lastly, the sidewalls are overturned from the spread-apart position to a closed position against the carcass ply and the ends of the tread band.
The method described is a source of a number of considerable shortcomings: in the first place, the step of spreading apart the carcass sidewall involves a radial stretching of the sidewall, perpendicularly to the tire rotation axis, with ensuing harmful dimensional variations of the semi-finished product, due to the plasticity of the material in the green state, which are very serious and sometimes unacceptable when the spreading operations of the sidewall are carried out manually. More particularly, the sidewall takes on an irregular wavy configuration predominantly concentrated along its periphery.
In addition, after the spreading apart operations, the free part of the sidewall no longer supported by the carcass wall in toric configuration, tends, due to its memory, to gradually return to a configuration free from internal stresses, i.e. a configuration corresponding to the deposition diameter of the sidewall onto the cylindrical surface of the assembly drum.
Consequently, there is a tendency of the radially external sidewall edge to curl axially outwardly on itself and against the radially internal portion which is already permanently fixed to the carcass ply and in any case to take on an irregular geometrical configuration in an axial direction.
This fact prevents the subsequent overturning of the sidewall in extended form against the carcass and the side portions of the tread band.
The operation is normally performed either manually or with the aid of rollers or through expansion of two small inflatable chambers arranged aside of the forming drum which, in their expansion, come into contact with the sidewall guiding its overturning against the carcass. In the situation just described, the sidewall overturning operation often originates a further undesired folding of the sidewall and in any case consolidates the curling and folding which may have possibly formed, which must be eliminated by using specific manual work with resulting cost increases of the manufacturing method and quality deterioration of the finished product.
Lastly, the removal of the anti-adhesive element, in particular when this is a thin sheet of plastics such as a strip of polyethylene is one of the most delicate operations in the tire manufacture, up to the point that the final result of this removal can determine the acceptability of the tire.
Indeed, it is to be noted that the anti-adhesive element is inserted between the sidewall and the carcass ply and thus creates with its radially internal edge the weld line between the sidewall and the carcass so that it may turn out to be as clamped between these elements.
In the removal step of the element consisting of the above mentioned sheet the forces applied by the operator may cause by tearing an entrapment of small sheet scraps between the sidewall and the carcass: the result is a welding failure between the rubber compositions separated by these scraps during the subsequent vulcanization step of the tire. The ensuing detachments in the tire may cause during operation a rapid destruction of the tire.
A last but no less important drawback, is that the generalized use of anti-adhesive elements of undegradable and unrecyclable materials, such as e.g. polyethylene, causes problems of environmental pollution due to the wasting of numberless releasing strips no longer reusable in the indicated production method.